Using first-principles density functional theory calculations combined with insight from a tight-binding representation, dynamical mean field theory, and linear response theory, we have extensively investigated the electronic structures and magnetic interactions of nine ferropnictides representing three different structural classes. The calculated magnetic interactions are found to be short range, and the nearest (J_{1a}) and next-nearest (J2) exchange constants follow the universal trend of J_{1a}/2J_{2} approximately 1, despite their itinerant origin and extreme sensitivity to the z position of As. These results bear on the discussion of itineracy versus magnetic frustration as the key factor in stabilizing the superconducting ground state. The calculated spin-wave dispersions show strong magnetic anisotropy in the Fe plane, in contrast with cuprates.

The electronic structures and transport properties of a series of actinide mono carbides, mono nitrides and dioxides are studied systematically using a combination of density functional theory and dynamical mean field theory. The studied materials present different electronic correlation strength and degree of localization of 5f -electrons, where a metal-insulator boundary naturally lies within. In the spectral function of Mott-insulating uranium oxide, a resonance peak is observed in both theory and experiment and may be understood as a generalized Zhang-Rice state. We also investigate the interplay between electron-electron and electron-phonon interactions, both of which are responsible for the transport in the metallic compounds. Our findings allow us to gain insight in the roles played by different scattering mechanisms, and suggest how to improve their thermal conductivities.

Using a novel many-body approach, we report lattice dynamical properties of UO2 and PuO2 and uncover various contributions to their thermal conductivities. Via calculated Grüneisen constants, we show that only longitudinal acoustic modes having large phonon group velocities are efficient heat carriers. Despite the fact that some optical modes also show their velocities which are extremely large, they do not participate in the heat transfer due to their unusual anharmonicity. Ways to improve thermal conductivity in these materials are discussed.Today's nuclear fuels are based on 235 U and 239 Pu elements where in a typical set-up, a nuclear reaction heats up a pellet made of either UO 2 or its mixture with PuO 2 and the heat is transformed to electrical energy. One of the major issues is to conduct the heat from a core of the pellet to its outer area which makes the evaluation of a high temperature thermal conductivity a key problem. Unfortunately, the thermal conductivity of UO 2 is very low and as a result, even a gradual increase of its value may lead to significant breakthrough in the performance of commercial nuclear reactors currently operating worldwide.In the present work, we argue that it is the unexpectedly large anharmonicity of optical modes that results in a very low thermal conductivity of modern nuclear fuels. Consider semiconducting UO 2 which is a main element of UOX fuel, or a blend of U and Pu oxides which is a major substance of MOX fuel. The heat from the core of the pellet in these insulating systems is carried by phonons which are known to be very inefficient heat conductors. This brings a whole set of complex problems such as causing fuel pellets to crack and degrade prematurely, necessitating replacement before the fuel has been depleted.Unfortunately, studying thermal conductivity In the present work we use a novel electronic structure method [13] capable of describing Mott insulating materials in order to address the structural properties and thermal conductivity of the modern nuclear fuels. Both UO 2 and PuO 2 are calculated using a combination of local density approximation (LDA) [12] and Dynamical Mean Field Theory (DMFT) [14][15] where relativistic 5f shells of Uranium and Plutonium atoms are treated by exact diagonalization of corresponding many-body Hamiltonians obtained by allowing a hybridization between the 5f -electrons and the nearest oxygen 2p orbitals.It is well known that a strong spin-orbit coupling of about 1eV present in actinides splits 14-fold degenerate f level onto f 5/2 and f 7/2 states. Group theoretical considerations assume that under cubic crystal symmetry, the f 5/2 6-fold degenerate level is further split onto Γ 8 quadruplet and Γ 7 doublet. In both UO 2 and PuO 2 , the Γ 8 level comes approximately 0.1eV below the Γ 7 state, and valence arguments make it occupied by two electrons for the case of UO 2 and fully occupied by four electrons for the case of PuO 2 . This sequence of the levels dictates the low temperature properties of these two materials: The UO ...

The physical properties and the band structure of the layered pnictide SrMnBi 2 were investigated. This compound has a crystal structure similar to that of the superconducting Fe pnictides, and is a bad metal with large residual resistivity. Magnetic order sets in at very high temperatures, around 290 K, as shown by magnetization, resistivity, and specific heat data. Band structure calculations using density functional theory (DFT) are consistent with the thermodynamic and transport measurements, suggesting a checkerboard antiferromagnetic (cAFM) ground state and a localized picture for the magnetism. Moreover, DFT results indicate that the Mn 3d electrons are strongly correlated, and that, unlike in the known superconductors, the Sr-Bi (1) layer is metallic. One more notable feature of the DFT calculation is the multiple Dirac-cone-like dispersion close to the Fermi level.

An efficient method of computing magnetic exchange interactions in systems with strong correlations is introduced. It is based on a magnetic force theorem that evaluates linear response due to rotations of magnetic moments and uses a generalized spectral density functional framework allowing us to explore several approximations ranging from local density functional to exact diagonalization based dynamical mean field theory. Applications to spin waves and magnetic transition temperatures of 3d metal mono-oxides as well as high-T(c) superconductors are in good agreement with experiment.

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Using a combination of local density functional theory and cluster exact diagonalization based dynamical mean field theory, we calculate many body electronic structures of several Mott insulating oxides including undoped high Tc materials. The dispersions of the lowest occupied electronic states are associated with the Zhang-Rice singlets in cuprates and with doublets, triplets, quadruplets and quintets in more general cases. Our results agree with angle resolved photoemission experiments including the decrease of the spectral weight of the Zhang-Rice band as it approaches k=0.Quasiparticle excitations in insulating transition metal oxides (TMOs) such as classical Mott-Hubbard systems or undoped high temperature superconductors (HTSCs) have been puzzling electronic structure theorists for many years [1,2]. While photoemission experiments in these materials show [3] the existence of the d-states located both right below the Fermi energy and at much higher binding energies (typically ∼10 eV), it is difficult to understand this genuine many-body redistribution of the spectral weight using calculations based on a static mean field theory [4,5], such as the density functional theory (DFT) in its local density approximation (LDA) [6]. Modern approaches, such as LDA+U [7], can differentiate between charge-transfer and Mott-Hubbard natures of these systems [8], but still have difficulties in recovering insulating behavior of the paramagnetic (PM) state and tackling more complicated many-body features such as Zhang-Rice (ZR) singlet of HTSCs [3,9]. Only most recent developments based on a combination of local density approximation (LDA) and dynamical mean field theory (DMFT) [10] have started to address those issues [11,12,13].In the present work, using a novel implementation of LDA plus cluster exact diagonalization based DMFT we demonstrate how to obtain accurate spectra of transition metal oxides and, in particular, describe full momentum dependent low-energy excitations associated in those systems with antiferromagnetic (AFM) Kondo-like coupling between a spin of oxygen hole injected by photoemission process and a local magnetic moment of the transition metal ion. These narrow energy bands are composed from the well known Zhang-Rice singlet states in cuprates [9], which have recently renewed their attention in connection with the disappearance of their spectral weight as the wave vector approaches the Brillouin Zone (BZ) center, and the observed high energy kink entitled as "waterfall" feature [14]. Zhang-Rice doublets have been discussed in NiO [15], and their further generalizations to triplets (CoO), quadruplets (FeO) and quintets (MnO) all naturally emerge from our LDA+DMFT calculations. We find that the ZR states exhibit a similar behavior in all systems including the loss of their spectral weight at the Γ point, which can be understood as the lack of hybridization between transition metal d states and neighboring oxygen p states, the effect most pronounced in HTSCs. There is a generally good agreement between o...

We show by quantum Monte Carlo simulations of realistic Kondo lattice models derived from electronicstructure calculations that multiple quantum critical points can be realized in plutonium-based materials. We place representative systems, including PuCoGa 5 , on a realistic Doniach phase diagram and identify the regions where the magnetically mediated superconductivity could occur. The solution of an inverse problem to restore the quasiparticle renormalization factor for f electrons is shown to be sufficiently good to predict the trends among Sommerfeld coefficients and magnetism. A suggestion on the possible experimental verification for this scenario is given for PuAs.